1. Technical Field of the Invention
The present invention relates to a spotlight mapping radar system which is provided on a radar platform attached to an air vehicle such as an artificial satellite, an aircraft and the like, and is adapted to obtain the image of a stationary object to be observed which is situated on the ground or surface of the sea by utilizing a beam stabilizing function.
2. Description of the Prior Art
The spotlight mapping radar system of this type according to the prior art is described in detail for example in Brookener, E. ed. "Radar Technology", chapter 18, Artech House., 1976. In this book, the constitution and the principle of this system is summarized as below.
FIG. 1 is a block diagram illustrating an embodiment of a spotlight mapping radar system according to the prior art. FIGS. 2 to 4 are intended to explain the operating principle and the working of the mapping system.
In FIGS. 1 to 4, numeral 1 designates a transmitter, numeral 2 a duplexer, numeral 3 an antenna, numeral 4 a receiver, numeral 5 a pulse compressor, numeral 6 an azimuth compressor, numeral 7 a display buffer, numeral 9 a display, numeral 10 a reference signal generator, numeral 11 an inertial navigation system, numeral 12 an antenna driving apparatus, numeral 13 a transmission signal, numeral 14 a received signal, numeral 15 an object to be observed, numeral 16 one spot in the object 15 to be observed, numeral 17 an antenna beam, and numeral 18 an air vehicle on which the spotlight mapping radar system is installed.
The antenna 3 is oriented by the antenna driving apparatus 12 toward the object 15 to be observed in accordance with the location and the speed of the air vehicle 18 which are measured by the inertial navigation system 11. Subsequently, the transmitted pulse signal generated by the transmitter 1 is emitted as a transmitted signal (in the form of a radio wave) by way of the duplexer 2 and the antenna 3 toward the stationary object 15 to be observed which is situated on the ground or surface of the sea. The transmitted signals 13 are reflected by the object 15 to be observed and received by the antenna 3 as the received signal 14 (or an echo signal). The received signal 14 is input into the receiver 4 by way of the duplexer 2. The receiver 4 amplifies the received signal 14 in the form of a radio frequency wave, detects it, and converts the detected signal into a video signal in a baseband. This video signal is input into the pulse compressor 5 and is processed by means of pulse compression in order to enhance the range resolution. The pulse compression enables the resolution in the direction perpendicular to the direction of flight or the distance (that is range) direction to be enhanced. In this case, the range resolution .DELTA.R is expressed by the equation: ##EQU1## (wherein C is the velocity of light and .tau. is the pulse width after pulse compression). Subsequently, the video signal is input into the azimuth compressor 6 so that the cross-range resolution in the flying direction or the azimuth direction of the air vehicle 18 can be enhanced. Azimuth compression is attained by utilizing the Doppler effect caused by movement of the air vehicle 18 with the spotlight mapping radar system installed thereon.
Suppose that the air vehicle 18 shown in FIG. 2 is moving in the rectilinear direction at a constant speed v, and one point 16 in the object 15 to be observed is located at the distance R.sub.0 and the squint angle .theta..sub.0 when the time t=0. At this instance, the relative distance R(t) between the air vehicle 18 and the point 16 at the time of t is expressed by the equation: ##EQU2## Accordingly, the instantaneous Doppler frequency fd(t) of the reception signal is provided by employing the transmission wave length of .lambda.; ##EQU3## As can clearly be seen from the third equation, the reception signal is a chirp signal in which the instantaneous Doppler frequency varies as the time t varies. In this case, since the parameters such as .lambda., R.sub.0, .theta..sub.0, v in the third equation are known or can be measured by the inertial navigation system 11, the history of variation of the Doppler frequency of the spot 16 corresponding to lapse of time or the Doppler history can be calculated by the reference signal generator 10. Accordingly, azimuth compression is made available for each range in a similar manner to pulse compression by correlating a series of reception signals with the reference signal generated by the reference signal generator 10.
In general, .theta..sub.0 is set as .theta..sub.0 =.pi./2 in a spotlight mapping radar system. At this time, the synthetic aperture time T is determined, not by the antenna beam width .theta..sub.B but by the variable range .theta..sub.S of the azimuth angle of the antenna driving apparatus 12 (in the case where the boresight direction of the antenna beam is movable in the range from (.pi.-.theta..sub.S)/2 to (.pi.+.theta..sub.S)/2 on the basis of the flying direction of the air vehicle 18) and is expressed by the equation; ##EQU4## Supposing that the synthetic aperture time is T, the crossrange resolution .DELTA.r can be expressed by the equation; ##EQU5##
Processed as above described, the reception signal of which resolution has been enhanced in both range direction and azimuth direction is stored in the display buffer 7 as the picture information and displayed on the display 9 as two dimensional radar picture images.
The required pulse repetition frequency PRF is expressed by the equation; ##EQU6##
As can clearly be seen from the fifth equation, the longer the synthetic aperture time T is, the more the crossrange resolution .DELTA.r may be enhanced. On the other hand, the observation range may be decided by the products R.theta..sub.B of the beam width .theta..sub.B and the distance R. Accordingly, if the observation time T which is identical to the synthetic aperture time T is equal to or less than the time when the radar platform (or the air vehicle) passes, that is R.theta..sub.B /v, the observation regions are consecutive as shown in FIG. 3. However, if the observation time T is extended to more than R.theta..sub.B /v, the observation regions become discontinuous as shown in FIG. 4. In order to prevent discontinuity of said observation regions; if the beam width .theta..sub.B is extended and the observation time T is also extended while the limitation of T.ltoreq.R.theta..sub.B /v is satisfied, then the pulse repetition frequency PRF must be made higher as can clearly be seen by the sixth equation, and as a result, range ambiguity is caused so that observation of a remote region is impossible.